Jørn Hurum, who led the expedition, compared the animal to a "medium-sized blue whale with a three-meter-long crocodile skull."

It was twice as big as a killer whale.

The pliosaur, a type of short-necked plesiosaur, was the leading marine predator during the Jurassic Period and is thought to represent an unknown species.

Its body was designed to minimize drag while its enormous flippers propelled it forward in a motion like flying through the water.

A front flipper from "The Monster" was measured at almost 10 feet long.

Fossil bones from the specimen excavated last summer showed that it was almost 50 per cent bigger than the largest confirmed pliosaur, Kronosaurus, from Australia.

An ichthyosaur, another marine predator, from 210 million years ago has previously been identified as being 75 feet long. but its teeth were much smaller than the pliosaur's and it would have chased much smaller prey.

"The pliosaur was much, much fiercer," said Hurum. "The ichthyosaur would have been an oversized fat dolphin by comparison. This animal would have taken chunks out of anything that moved. It was the fiercest marine reptile and the biggest of its era. Its teeth and jaws could crush almost anything."

Dr. Patrick Druckenmiller, a plesiosaur specialist at the University of Alaska Museum, was involved in the discovery.

"Not only is this specimen significant in that it is one of the largest and relatively complete plesiosaurs ever found," said Druckenmiller, "it also demonstrates that these gigantic animals inhabited the northern seas of our planet during the age of dinosaurs."

In 2002 a fossil pliosaur from Mexico was nicknamed the Monster of Aramberri, amid claims that it was up to 65 feet long. However, the measurements have yet to be confirmed.

Angela Milner, associate keeper of paleontology at the Natural History Museum in London, said the find illustrated how different the world was when the animal ruled the seas.

"Svalbard was not so near the North Pole 150 million years ago — there was no ice cap and the climate was much warmer than it is today."

She added: "There are a few isolated bones of huge pliosaurs already known, but this is the first find of a significant portion of a whole skeleton of such a giant."

It has been suggested that the Loch Ness Monster could be a long-necked plesiosaur.

Skeptics, however, have pointed out that the loch is 10,000 years old, whereas plesiosaurs are thought to have died out 65 million years ago.